1. Statement of the Technical Field
The present invention relates generally to RFID transponder tags and more particularly to RFID transponder tags which are tolerant to electrical loading associated with adjacent materials.
2. Description of the Related Art
A known type of electronic article surveillance system includes a transmitter-receiver arrangement disposed alongside an area to be controlled. In such systems, a transmitter transmits a first signal into the controlled area. For example, the first signal can be an electromagnetic radio frequency (RF) signal in the UHF range. A separate transmitter produces a second signal within the same area. The second signal is usually an electric field of a substantially lower frequency and is commonly referred to as the E-field or electrostatic field. Re-radiators, typically comprising a dipole and a nonlinear element, are responsive to the first and second signals to produce RF signal at certain mixing frequencies which are derived from the first signal and the second signal. The RF signals present at one or both of the mixing frequencies can be detected by the receiver and used as a basis of detecting the presence of the re-radiator within the detection area. The RF frequencies produced by a mixing type re-radiator will generally include signals at the sum and difference of the first and second signal which are incident upon the device. As an example, a 900 MHz UHF signal and a 250 KHz E-field will produce a mixing component at 900.250 MHz and 899.750 MHz. An exemplary system similar to the type described herein is disclosed in U.S. Pat. No. 8,013,742 to Lian et al.